Tab Previews was originally to be introduced in Firefox 3.0 and 3.5, but backed out of both. The Firefox 3.6 Alpha 1 nightly build now includes tab previews, just type about:config in the address bar, search for browser.ctrlTab.previews and change the value to true.
There is also a new behavior in tab previews, instead of moving to the next tab in the tab bar, you can also move to the previously used tab. To enable this new behavior, search for browser.ctrlTab.mostRecentlyUsed and change the value to true.
So what does tab preview show and how does it work? Press Ctrl+Tab to bring up tab preview, you will see the current open along with five most recently used tabs.
There is also an option to show all tabs thumbnails, which includes a search bar that can be used to search for keywords in title tags.
It is still in early development stage and has a long way to go before it gets fully themed for every OS. Enjoy!